The present invention relates to linearization of frequency sweep systems. More specifically, the present invention relates to a marker sweep linearization system for linearizing the sweep of a voltage controlled oscillator over a selected frequency range.
In present signal generation and processing systems, it is often necessary to produce a signal whose frequency can be varied or swept over a prescribed range. Typically, this requires accurate tracking of signal frequency to a selected sweep control input. In many signalling applications, such as radar, spectrum analyzers, etc. the requisite sweep must be linear with time, i.e., a linear frequency ramp, to cover the frequency range of interest. Generally, a voltage controlled oscillator is used to produce a signal whose frequency can be varied or swept over a prescribed range, since it produces an output frequency that is directly related to an input control voltage. Unfortunately, voltage controlled oscillators are inherently nonlinear (i.e., oscillator frequency is not a linear function of sweep voltage). Further, voltage controlled oscillators tend to drift unpredictably with time and temperature.
Prior art linearization circuits are described in U.S. Pat. Nos. 4,129,832; 4,499,435: 4,904,964; and 4,998,217 which are discussed more fully below. The above list is only exemplary and is not intended to be a complete list of the prior art.
U.S. Pat. No. 4,129,832 to Neal et al discloses a linearization system for voltage controlled oscillator sweep generators wherein a ramp signal is preprogrammed to control the input voltage of the oscillator. A difference frequency is compared to a reference frequency to determine a required adjustment to the ramp signal.
U.S. Pat. No. 4,499,435 to Thomson et al discloses a system for linearizing sweep of a voltage control oscillator employing both a calibration mode and a measurement mode. In the calibration mode a sweep rate of the voltage control oscillator is slowed down (as it is known that voltage control oscillators have greater linearity at lower frequencies) and the signals processed to define an intended ramp which is linear with time.
U.S. Pat. No. 4,904,964 to Peng et al discloses a voltage control oscillator circuit utilizing modulation compensation to yield a constant modulated level over a frequency band. The capacitance for coupling the modulating signal to a compensated input of the voltage control oscillator is varied to compensate the input of the voltage control oscillator.
U.S. Pat. No. 4,998,217 to Holcomb et al discloses a sweep generator linearization system. The system has two modes of operation, a calibration mode and a measurement mode. In the calibration mode the voltage controlled oscillator is locked at low, center and high frequencies and the corresponding voltages are measured. This information is utilized by a microprocessor to linearize the sweep. During the measuring mode two sweeps are made at different rates to assure that the center frequency occurs at the midpoint of the sweep cycle. When the microprocessor determines that the shift in the center frequency in time from the midpoint of the sweep cycle is not acceptable the system can be switched back to the calibration mode, as described above.